In recent years, studies on gas sensors that detect CO2, for example, have been made at the requests for environmental monitoring. As a gas sensor of this kind, an infrared light absorption type gas sensor (NDIR: Nondispersive infrared detector) is known (for example, refer to Non Patent Literature 1). Further, as another gas sensor, a gas sensor using surface plasmon resonance (SPR) is known (for example, refer to Non Patent Literature 2).